Just as the elucidation of the structure of DNA provided valuable insights into the way the genetic material was replicated and information encoded, we believe that the solving of the structure of the eukaryotic chromosome will provide analogous types of insights into the way genes are regulated and chromosomes are replicated in higher organisms. We therefore wish to continue our analysis of the structure of the eukaryotic chromosome. This analysis will focus on four primary points: (1) the structure of nucleosomes, (2) the structure of chromatin that is actively being transcribed, (3) the in vivo mechanism of assembly of newly replicated DNA into chromatin and (4) the role of histone H1 in determining higher order packaging of nucleosomes. We believe that the clue to our understanding of eukaryotic gene regulation lies in the understanding of how histones, in conjunction with other nuclear elements, generate a large number of chromosomal subunits. This requirement to form many types of interactions can explain why the amino acid sequences of the majority of histones are so rigidly conserved.